1. Field of the Invention
This invention relates to the field of gain control stages, particularly those implemented with a differential pair.
2. Description of the Related Art
Automatic gain control (AGC) circuitry is found in many wireless communication devices, including radios, microwave transceivers and cellular phones. Some of the functional blocks found in these devices, such as IF amplifiers and analog-to-digital converters, perform best when presented with an input having a nearly constant signal strength. However, the variability in the strength of RF signals received by the devices normally causes the strength of these inputs to vary. AGC circuitry, usually placed in the front end of the functional blocks in which it is needed or in its own discrete block, serves to level out the signal strengths of these inputs. The performance advantages provided by the circuitry nearly always outweigh the complexity it adds to the device or block in question. Conventional applications of AGC circuitry are discussed, for example, in Rohde, Communications receivers: principles and design, McGraw-Hill, Inc. (1988), pp. 238-246.
A differential pair can be used as an element of an AGC. A differential pair consists of two transistors which are joined at a common junction formed by coupling the emitters of a bipolar pair or the sources of a FET pair, with degeneration resistors sometimes placed between their respective emitter (or source) terminals and the common junction. As shown in FIG. 1, one gain control stage 10 is implemented with a differential pair of npn transistors QA and QB. An input current signal I.sub.in is connected to the pair's common emitter junction, and a differential control signal C+ and C- is connected to the pair's respective bases. The stage's current output I.sub.out is taken at the collector of QA, with the collector of QB connected to a fixed supply voltage V.sub.supply. When C+ is greater than C-, QA conducts more current than QB and the stage's gain, i.e., I.sub.out /I.sub.in is greater than 0.5; when C- is greater than C+, QB conducts more than QA and the stage's gain is less than 0.5. As used herein, "gain" refers to both the amplification (gain &gt;1) and attenuation (gain &lt;1) of a signal. With the gain control stage of FIG. 1 there can only be attenuation or unity gain, since I.sub.out cannot exceed I.sub.in. The gain can be varied between about 0.01 and about 1; base currents through QA and QB make gains of exactly 0 or 1 practically unattainable.
The collector currents of a bipolar differential pair vary with the pair's differential input voltage over 8V.sub.T (V.sub.T 18 26mv at 25.degree. C.), or about 208 mv; beyond this the differential pair transfer function displays a saturated response and further increasing the input differential voltage does not appreciably change the collector currents. The differential input voltage signal C+ is typically made to swing symmetrically above and below C- by about 4V.sub.T. When C+ is 4V.sub.T above C-, nearly all current I.sub.in flows through QA and the stage's gain .apprxeq.1; when C+ is 4V.sub.T below C-, nearly all current flows through QB and the gain .apprxeq.0. Between these extremes, i.e., over most of the differential input voltage signal range, both QA and QB are conducting.
A drawback to the use of a differential pair for gain control is that the pair introduces noise into the signal path, primarily caused by QA's base shot noise, base thermal noise and collector shot noise; the noise contribution from these sources increases when QA's emitter looks into a low impedance, as is presented when QB is conducting. Thus, the gain control stage degrades noise figure when driven by a symmetrical control signal. The noise performance of a differential pair is discussed, for example, in Gray and Meyer, Analysis and Design of Analog Integrated Circuits, John Wiley and Sons, Inc. (1984), pp. 679-681.
Another functional block found in most wireless communications devices is a mixer, which converts an incoming high-frequency RF signal down to an intermediate frequency (IF) which can be handled by downstream signal processing circuits. Gilbert multiplier cells, which are discussed, for example, in Gray and Meyer, supra, pp. 590-605,are often configured to perform this function; a Gilbert multiplier cell is referred to as a Gilbert mixer when so configured.
A basic Gilbert mixer circuit configured as a downconverter is shown in FIG. 2. The differential voltage components RF+ and RF- of an incoming RF signal are connected to the respective bases of a differential pair of transistors Q1 and Q2, which form the mixer's RF input stage 12 and are biased to operate in their linear region with a current source I1 connected to their common emitter junction. The collector currents I.sub.rf1+ and I.sub.rf1- of Q1 and Q2 vary in accordance with the combination of RF+ and RF-.
The mixer has an output stage 14 consisting of two differential pairs, with the respective collectors of a differential pair Q3/Q4 connected to the respective collectors of a differential pair Q5/Q6. A local oscillator signal (LO) made up of complementary differential voltage components LO+ and LO- is applied to the two pairs, with LO+ connected to the bases of Q3 and Q6, and LO- connected to the bases of Q4 and Q5. The common emitter junction of Q3 and Q4 conducts current I.sub.rf1+ and the common emitter junction of Q5 and Q6 conducts current I.sub.rf1-.
Local oscillator signal LO is preferably a 50% duty cycle square wave, and the transistors Q3-Q6 controlled by LO function as switches: when LO+ is high (and LO- is low), Q3 and Q6 are turned on and Q4 and Q5 are off, and when LO+ is low, Q4 and Q5 are turned on and Q3 and Q6 are off. Thus, the flow of I.sub.rf1+ alternates between switches Q3 and Q4 and the flow of I.sub.rf1- alternates between switches Q5 and Q6, in accordance with the state of signal LO.
The current output of the mixer is a signal IF1 made up of differential current components IF1+ and IF1-, which are taken at the collectors of Q3/Q5 and Q4/Q6, respectively. The frequency spectrum of the output signal includes (but is not limited to) components at the sum and difference frequencies, i.e., RF+LO and RF-LO, with RF-LO usually being the down-converted signal of interest (when the mixer is configured as a down converter).
A mixer's output is typically connected to signal processing circuits which, as noted above, are designed to provide optimum performance when processing signals having a nearly constant signal strength. However, the output signal strength of the Gilbert mixer shown in FIG. 2 varies with the strength of the incoming RF signal. This inability to provide a constant signal strength output is a shortcoming which results in the overall performance of the communication device being degraded.
Another problem of the Gilbert mixer shown in FIG. 2 is caused by the junction capacitances inherently found between the collector and base junctions of transistors Q1 and Q2. The transitioning of the LO signal from high to low and vice versa can cause noise and voltage spikes to appear at the respective common junctions of differential pairs Q3/Q4 and Q5/Q6. The junction capacitances of Q1 and Q2 couple the spikes to the RF+ and RF- input signals, respectively, and these input-distorting spikes also pass through the output switches and appear in the mixer's IF1 output.
One remedy for this junction capacitance-induced problem is shown in FIG. 3. Transistors Q7 and Q8 are connected in a cascode configuration between RF input transistors Q1 and Q2 and differential pairs Q3/Q4 and Q5/Q6, respectively, to form an LO/RF isolation stage. Q7 and Q8 both receive a constant base bias voltage V.sub.BIAS that cause them both to operate linearly with about unity gain. Voltage spikes that previously were coupled to the RF input are now shunted to the bias circuit. However, due to the voltage drop of between about 0.5 and 0.9 volts introduced into the signal path by Q7 and Q8, use of an isolation stage requires more supply voltage headroom. A higher voltage power supply can provide the additional headroom, but this increases the device's power dissipation, which is particularly undesirable for these mostly battery-powered devices.